SLCDs as defined in the preamble are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Arpl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term SLCD here covers any more highly twisted display element with a value for the twist angle of between 160.degree. and 360.degree., such as, for example, the display elements of Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
SLCDs of this type are distinguished, in comparison to standard TN displays, by significantly better steepnesses of the electrooptical characteristic line and consequently better contrast values, and by significantly less angle dependence of the contrast. Of particular interest are SLCDs having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures were hitherto optimized using usually monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for all applications.
In order to achieve a steep electrooptical characteristic line in SLCDs, the liquid-crystal mixtures should have relatively large values for K.sub.33 /K.sub.11 and relatively small values for .DELTA..di-elect cons./.di-elect cons..sub.195 .
In addition to optimization of the contrast and the response times, further important requirements are made of mixtures of this type:
1. A broad d/p window PA1 2. High long-term chemical stability PA1 3. High electrical resistance PA1 4. Low frequency and temperature dependence of the threshold voltage. PA1 R.sup.a is alkyl, alkoxy, alkenyl or alkenyloxy having 1 to 12 carbon atoms, PA1 R.sup.b is alkyl or alkoxy having 1 to 5 carbon atoms, PA1 Z is --COO--, --CH.sub.2 CH.sub.2 -- or a single bond, and ##STR8## PA1 are each, independently of one another, ##STR9## PA1 one of the rings ##STR10## PA1 may alternatively be ##STR11## PA1 R.sup.3 is an alkenyl group having 2 to 7 carbon atoms, PA1 R.sup.4 is as defined for R.sup.a in the formula IA, and PA1 c is 0 or 1. PA1 they have low viscosity, PA1 they have low temperature dependence of the threshold voltage and the operating voltage. PA1 two outer plates which, together with a frame, form a cell, PA1 electrode layers with alignment layers on the insides of the outer plates, PA1 a pretilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 1 degree to 30 degrees, and PA1 a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 100.degree. and 600.degree., and PA1 a nematic liquid-crystal mixture of positive dielectric anisotropy which is present in the cell, consisting of PA1 characterized in that the liquid-crystal mixture comprises one or more compounds of the formula IA ##STR13## PA1 R is an alkyl, alkoxy or alkenyl group having 1 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH.sub.2 groups may be replaced by --O--, --CH.dbd.CH--, --CO--, --OCO-- or --COO--, in such a way that O atoms are not linked directly to one another, ##STR27## PA1 are each, independently of one another, ##STR28## PA1 Z.sup.1 is --COO--, --CH.sub.2 CH.sub.2 or a single bond, PA1 Z.sup.2 is --CH.sub.2 CH.sub.2, --COO--, --C.tbd.C-- or a single bond, PA1 Q is --CF.sub.2 --, --CHF--, --CH.sub.2 --, --OCF.sub.2 --, --OCHF-- or a single bond, PA1 Y is F or Cl PA1 a is 1 or 2, and PA1 b is 0 or 1. PA1 R is an alkyl, alkoxy or alkenyl group having 1 to 12 carbon atoms, in which, in addition, one or two non-adjacent CH.sub.2 groups may also be replaced by --O--, --CH.dbd.CH--, --CO--, --OCO-- or --COO--, in such a way that O atoms are not linked directly to one another ##STR32## PA1 and Q.sup.1 are each, independently of one another, ##STR33## PA1 Z.sup.3 is ##STR34## PA1 or a single bond PA1 d is 0 or 1, PA1 L.sup.1 to L.sup.6 are each, independently of one another, H or F, PA1 Q is --CF.sub.2 --, --CHF--, --CH.sub.2 --, --OCF.sub.2 --, --OCHF-- or a single bond, PA1 Y is F or Cl, PA1 Z.sup.4 is --CO--O--, --CH.sub.2 CH.sub.2 -- or a single bond, and PA1 R.sup.1 and R.sup.2 are each, independently of one another, as defined for R in the formulae II and III. PA1 R is as defined under the formulae II and III, PA1 Z.sup.5 is --CH.sub.2 CH.sub.2 --, --CO--O-- or a single bond, PA1 Q2 is ##STR58## PA1 alkyl is an alkyl group having 1 to 9 carbon atoms, PA1 X is CN or F, and PA1 L is H or F, PA1 R.sup.1 and R.sup.2 are each, independently of one another, as defined for R in the formulae II and III, PA1 Y is F or Cl, and ##STR60## PA1 R.sup.1a is CH.sub.3 --(CH.sub.2).sub.p --, CH.sub.3 --(CH.sub.2).sub.p --O--, CH.sub.3 --(CH.sub.2).sub.p --O--CH.sub.2 --, trans-H--(CH.sub.2).sub.q --CH.dbd.CH--(CH.sub.2 CH.sub.2).sub.s -- or trans-H--(CH.sub.2).sub.q --CH.dbd.CH--(CH.sub.2 CH.sub.2).sub.s --CH.sub.2 O--, PA1 R.sup.2a is CH.sub.3 --(CH.sub.2).sub.p --, PA1 p is 1,2,3 or 4 PA1 q is 0,1,2, or 3, and PA1 s is 0 or 1. PA1 B2I: from about 5 to 30%, preferably from about 5 to 15%, sum of B2II PA1 and B2III: from about 5 to 25%, preferably from about 10 to 20%. PA1 one or more compounds of the following formulae ##STR66## PA1 one or more, in particular 1, 2, 3 or 4, compounds selected from the compounds of the formulae IIIb, IIId, IIIf, IIIh, IIIi, IIIs and IIIu; PA1 at least two compounds selected from the compounds of the formulae IIb1, IIb2, IIb3, IIc1 and IIc2. The proportion of these compounds in the liquid-crystal mixtures is preferably from 0 to 60% by weight, particularly from 10 to 45%; PA1 one or more compounds of the formula T1 to T4, in particular one or more compounds of the formula T2a and/or T3a, where the proportion of these compounds in the liquid-crystal mixtures is preferably from 0 to 25%, in particular from 1 to 15%. PA1 at least two compounds of the formula AI or AII; PA1 one or more compounds in which R or R.sup.1 is a trans-alkenyl group of trans-alkenyloxy group; PA1 one or more compounds selected from the following group: ##STR67## PA1 in which R.sup.1 and R.sup.2 have the preferred meanings given under compounds of component B, and where the compounds of the formula IV6 are different from the compounds of the formula IB. The 1,4-phenylene groups in the abovementioned compounds can also be substituted by fluorine; PA1 one or more compounds of the formulae ##STR68## PA1 in which R, R1 and R2 are each, independently of one another, as defined for R in the formulae II and III.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex, but also for low- and medium-multiplex STNs (1/400). This is in some cases attributable to the fact that the various requirements are affected in opposite manners by material parameters.
There thus continues to be a great demand for SLCDs, in particular for high-resolution displays (XGA), having very short response times and at the same time a large operating temperature range, high characteristic line steepness, good angle dependence of the contrast and low threshold voltage which meet the abovementioned requirements.